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By JOHN EMSLEY THREE new molecules, each with the chemical formula of benzene but a different arrangement of atoms have been made in Texas. This brings the total number of forms, or isomers, of benzene to seven. More importantly, it completes the set of all possible isomers with the chemical formula C6H6. Benzene is the only stable arrangement of six carbon atoms and six hydrogen atoms. In this molecule, single bonds alternate with double bonds. Although chemists have known its structure for more than a century, benzene was ‘photographed’ only comparatively recently. In 1988, scientists at IBM’s Almaden Research Center in San Jose, California, used a scanning tunnelling microscope to show conclusively that benzene’s six carbon atoms are arranged in a ring. Michael Faraday discovered benzene 165 years ago in cylinders that had held acetylene gas. It was later named by August Kekule, who proposed its structure in 1865. He claimed that the idea came to him one summer evening as he dozed on a Clapham omnibus, but this claim is now being disputed. In 1867, James Dewar proposed a second structure (see Diagram), which was proved in 1963 when his molecule was finally made. Chemists have also proposed other structures, such as benzvalene, which was isolated in 1967, and prismane, which was prepared in 1973. The only remaining isomers of C6H6 that were possible were the double-ring systems known as bicyclopropenyls. There are three isomers: 1,1; 1,2; and 2,2. They differ only in the positions of their double bonds.FIG-mg17043201.GIF Now, Wilbur Billups and Michael Haley of Rice University in Houston have made the 2,2 and 1,2 isomers of bicyclopropenyl. They also claim to have made the 1,1 version, but give no details (Angewandte Chemie, International Edition in English, vol 28, p 1711). Billups and Haley synthesised the bicyclopropenyl (2,2) compound from 1,4-bis(trimethylsilyl)buta-1,3-diene. First, they reacted this substance with methyl lithium and dichloromethane. Then they reacted it with tetra-n-butylammonium fluoride. This resulted in a yield of 45 per cent of the bicyclopropenyl (2,2) derivative. To identify the new isomer, Billups and Haley used nuclear magnetic spectroscopy. In this technique, scientists use a tuned magnetic field to identify the six hydrogen atoms by the spin of their nuclei. Bicyclopropenyl (2,2) is stable only below 10 Degree C; above this temperature, it slowly decomposes to a yellow solid, as yet unidentified. Billups and Haley were surprised by this, because they expected it to revert to benzene itself. They calculated that this would release a remarkable 500 kilojoules per mole of energy. The question of who really came up with the structure of benzene has received much attention lately. William Wiswesser, of the US Department of Agriculture, says that the honour belongs to Johann Loschmidt, a little-known Viennese chemist (Aldrichimica Acta, vol 22, p 17). Four years earlier than Kekule, Loschmidt proposed a ring of six carbon atoms for the structure. He published his idea in a private pamphlet. According to Wiswesser, Kekule certainly knew of Loschmidt’s proposal through a mutual friend,